Generally, a spark plug has on its forward end side a center electrode and a ground electrode and has on its rear end side a metallic terminal for receiving power supply. The metallic terminal protrudes from the rear end of an insulator, and the insulator is accommodated and held within a metallic shell. In a spark plug production process, a crimping step is performed, i.e., the insulator is inserted into the tubular metallic shell, and a to-be-crimped portion at the rear end of the metallic shell is crimped to fix the insulator (for example, see Japanese Patent Application Laid-Open (kokai) No. 2013-101805). The metallic shell includes a thick-walled tool engagement portion and a thin-walled to-be-buckled portion (which may be also referred to as a “thin-walled portion”) that are disposed forward of the to-be-crimped portion, and the to-be-buckled portion buckles in the crimping step. The crimping step is performed using a crimping press and is therefore referred to also as a “crimping-pressing step.”
The amount of buckling of the to-be-buckled portion in the crimping-pressing step is a major factor that determines the state of fixation between the insulator and the metallic shell and the positional relation between the metallic terminal and the metallic shell, so that the amount of buckling has a large influence on the performance (particularly the durability and ignition performance) of the spark plug. Therefore, it is desired to adjust the amount of buckling in the crimping-pressing step to be as close as possible to a predetermined target buckling amount. The amount of buckling depends directly on the amount of movement of a jig of the crimping press (which is referred to as a “crimping jig”) that is pressed against the to-be-crimped portion of the metallic shell in the crimping-pressing step. Therefore, it is desired to adjust the moving distance of the crimping jig in the crimping-pressing step to be as close to as possible to a predetermined target moving distance. Particularly, in a small-diameter spark plug in which a so-called insulator mark diameter (the outer diameter of the insulator at the rear end of the metallic shell) is small, the wall thickness of the to-be-crimped portion of the metallic shell is small, so that the above issue is particularly important.